Method for suppressing narrowband noise in a wideband communication system

ABSTRACT

This invention is used in communications, for example, in expanded signal spectrum broadband communication systems. The technical effect of this invention consists in an enhanced narrow-band interference suppression factor and almost complete elimination of the effect displayed by a powerful narrow-band interference or a group of narrow-band interferences in a limited frequency band. A noise signal formed in the frequency band (F 0 , F 1 ) in the transmission channel of a broadband communication system is power-modulated by a given modulation technique at a modulation frequency F mod &lt;&lt;(F 1 −F 0 ) and passed through a propagation medium, in which a narrow-band interference is superimposed thereon; received in the receiver; filtered in the frequency band (F 0 , F 1 ); amplified, and divided into two signals. One of the signals is obtained by amplifying the filtered signal and limiting the amplitude thereof, and the other signal is the filtered signal or a signal linearly amplified without altering the shape thereof. The two signals obtained are then multiplied; the resultant signal is filtered in the frequency band [ΔF nar , (F 1 −F 0 )]; and the envelope of the signal obtained by filtration in the frequency band [ΔF nar , (F 1 −F 0 )] is separated in order to be subsequently demodulated and to give an information signal.

(i) SCOPE OF THE INVENTION

This invention relates to radio engineering and can be used incommunication equipment, for example, in expanded signal spectrumbroadband communication systems.

(ii) PRIOR ART OF THE INVENTION

Conventional systems expand the signal spectrum by modulating signalamplitude, phase and frequency, or a combination of these. Narrow-bandinterferences, that is, interferences having all energy thereofconcentrated within a narrow frequency band, are the main kind ofinterference. Filtering a composite signal in a receiver experiencingthe effect of fluctuating noise and powerful narrow-band interferencesin a desired signal spectrum is, therefore, a very insistent task.Action to deal with it is called for by the fact that a powerfulnarrow-band station can disrupt communications in broadband systemscompletely, because the correlator multiplier converts a narrow-bandinterference into a pulse-phase signal having a linear spectrum with anenvelope proportional to sin(x)/x, so that in the presence of a powerfulenough interference, noise level at the correlator output can exceedthat of the convolved desired signal.

There are many methods to correct this deficiency, for example, byquasi-optimal linear filtration, trapping of a spectrum section,compensation techniques, and so on (1).

These prior art methods are deficient because of their complexity.

The closest prior art of this invention is a method of suppressingnarrow-band interferences in a broadband communication system, whereinthe transmitting channel is used to generate a desired signal to betransmitted; the pseudorandom signal frequency cycle parameter F_(prp)is altered in accordance with given information; a digital phase noisesignal is formed at a central frequency F₀ and logically summed up withthe digital pseudorandom signal formed previously; the resultant signalis amplified in the receiver; the signal transmitted is receivedtogether with a narrow-band signal superimposed thereon in thepropagation medium in the frequency band F_(nar)<<F_(prp); the signalreceived is filtered by passing it through a high-frequency bandpassfilter at a pass band frequency 2F_(prp) and central frequency F₀; theinput signal is converted into a voltage proportional to power; theresultant signal is filtered in the frequency band (F₀−F_(prp)) andamplified; the resultant signal is limited; and a correlation processingmethod is then used to obtain an error signal as the difference betweenthe clock frequency F_(prp) of the signal being transmitted and therespective clock frequency of the receiver (2).

The prior art method is disadvantageous because of its low interferencesuppression factor when interferences are generated by powerfulnarrow-band stations.

(iii) DISCLOSURE OF THE INVENTION

The technical effect of this invention consists in raising thenarrow-band interference suppression factor in the receiver and almostcompletely eliminating the influence of a powerful narrow-bandinterference or a group of narrow-band interferences in a limitedfrequency band, including frequency-modulated and scanninginterferences, as a result of which communication quality is improved byenhancing noise immunity of the desired signal.

This technical result is achieved by a method of suppressing anarrow-band interference in a broadband communication system, wherein abroadband noise signal is generated in the transmission channel in afrequency band (F₀, F₁); the broadband noise signal is modulated by agiven modulation technique for modulating power at a modulationfrequency F_(mod)<<(F₁−F₀); the resultant signal is passed through apropagation medium and received in the receiver together with anarrow-band interference superimposed thereon in the propagation medium,and filtered in the frequency band (F₀, F₁); two signals are generated,one of which is obtained by amplifying a signal filtered in thefrequency band (F₀, F₁) and limiting the amplitude thereof, and theother signal is the above filtered signal or a filtered signal linearlyamplified without altering the shape thereof; the two signals thusobtained are multiplied; the resultant signal is filtered in a frequencyband [ΔF_(nar), (F₁−F₀)}; the envelope of the resultant signal isselected and demodulated to obtain an information signal, whereinΔF_(nar) is the frequency band of the squared amplitude variationspectrum of the narrow-band interference voltage.

(iv) PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a block diagram of an apparatus designed to implement thepresent method of suppressing a narrow-band interference in a broadbandcommunication system;

FIG. 2 shows a block diagram of a propagation medium;

FIG. 3 shows the shape of a signal filtered in the frequency range (F₀,F₁), which is received following its amplification and limitation;

FIG. 4 shows the shape of a signal filtered in the frequency band (F₀,F₁) or a linearly amplified signal filtered in the frequency band (F₀,F₁);

FIG. 5 shows the spectrum of a resultant signal obtained by multiplyingthe above two signals; and

FIG. 6 shows the spectrum of a resultant signal filtered in thefrequency band [ΔF_(nar), (F₁−F₀)].

An apparatus used to carry out the present method of suppressing anarrow-band interference in a broadband communication system comprises atransmission channel containing a broadband noise signal generator 1connected in series to a modulator 2 and a transmitting antenna 3 (FIG.1), from which a signal is transmitted through a propagation medium(FIG. 2) to the receiving antenna of a receiver 4 that applies thesignal to the input of a bandpass filter 5 having a frequency band (F₀,F₁), the output of which is connected to the input of an amplifier 6.The output of amplifier 6 is connected to the inputs of a linearamplifier 7 and a limiting amplifier 8, which have outputs connected tothe inputs of a multiplication unit 9 having an output connected to theinput of a bandpass filter 10 having a frequency pass band [ΔF_(nar),(F₁−F₀)]. The output of bandpass filter 10 sends a signal to the inputof a unit 11 separating the signal filtered in the frequency band[ΔF_(nar), (F₁−F₀)]. The resultant signal is applied to a demodulator 12to separate an information signal.

The present method of narrow-band interference suppression in abroadband communication system is performed as follows:

Generator 1 of the transmission channel produces a broadband noisesignal in the frequency band (F₀, F₁) that is power-modulated (inmodulator 2) by a given modulation technique at a modulation frequencyF_(mod)<<(F₁−F₀). The resultant signal is transmitted to a propagationmedium, such as radio ambient (FIG. 2), where a narrow-band interferenceis superimposed on it. The narrow-band interference superimposed on thebroadband noise signal in the propagation medium may beamplitude-modulated, frequency-modulated, scanning, and so on, but it isrequired to fulfill the following conditions: the narrow-bandinterference frequency F_(nar) must fulfill the condition F₀<F_(nar)<F₁,and the frequency band of the squared amplitude variation spectrum ofthe interference voltage (ΔF_(nar)) must be much smaller than thefrequency band of the squared amplitude variation spectrum of themodulated broadband noise signal voltage at the reception point. Thereceiver input is, therefore, supplied with a signal equal to the vectorsum of a desired signal voltage U_(sig) and a narrow-band interferencevoltage U_(nar). This mixed signal is applied to the input of bandpassfilter 5 having a frequency pass band (F₀, F₁). Then the filtered signalpre-amplified in amplifier 6 is divided into two signals. The firstsignal is obtained after the signal filtered in the above frequencyband, and its amplitude limited, has been amplified in amplifier 8. Asthe signal passes, together with the interference, through the limitingamplifier the interference suppresses the desired signal, and theamplifier output gives a normalized signal U_(nar)/|U_(nar)|. The secondsignal is the above filtered signal or a filtered signal amplified inlinear amplifier 7. Linear amplifier 7 does not alter the shape of thefiltered signal, which will have the form of k·(U_(sig)+U_(nar))|. Theshapes of these two signals are shown in FIGS. 3 and 4, respectively.The two signals obtained as above are applied to respective inputs ofthe multiplication unit, which multiplies them to give at the outputthereof a resultant signal of a spectrum illustrated in FIG. 5. Theresultant signal is then filtered in bandpass filter 10 having afrequency pass band [ΔF_(nar), (F₁−F₀)]. In this case, the narrow-bandinterference serves as a heterodyne for desired signals, and if thecondition, under which the spectrum frequency band ΔF_(nar) is muchsmaller than the frequency band of the squared amplitude variationspectrum of the modulated broadband noise signal voltage, is fulfilled,the narrow-band interference itself is eliminated after the resultantmultiplication signal has been passed through the bandpass filter havinga frequency pass band [ΔF_(nar), (F₁−F₀)]. Then, by separating theenvelope from the filtered signal, it is possible to obtain a desiredpower-modulated signal, which is then processed by common demodulationtechniques to obtain an information signal (FIG. 6). Moreover, powermodulation F_(mod) can be effected by various methods, for example,using amplitude-frequency modulation or pulse modulation and anyencoding methods and pseudorandom sequences.

Information is, therefore, incorporated in signal power variation andtransmitted over the entire frequency band (F₀, F₁), and when thespectrum is transferred by processing in the receiver, it is transferredtogether with the spectrum.

It follows, therefore, that the spectral band of interference powervariation, rather than the frequency band of the interference in theambient is the limiting factor for this method of suppressing anarrow-band interference, making it even possible to suppress a scanninginterference without knowing its actual location in the ambient.

REFERENCES

1. “Address Control and Communication Systems,” edited by G. I. Tuzov,Radio & Communications, Moscow, 1993, pp. 256–259 and 261–264.

2. RU 2,127,021, C1, (V. V. Kalugin et al.), Feb. 27, 1999.

1. A method of suppressing a narrow-band interference in a broadbandcommunication system, wherein a broadband noise signal is formed in thetransmission channel in a frequency band (F₀, F₁); the broadband noisesignal is modulated by a given modulation technique to modulate thepower thereof at a modulation frequency F_(mod)<<(F₁−F₀); the resultantsignal is passed through a propagation medium and received by thereceiver together with a narrow-band interference superimposed thereonin the propagation medium, and filtered in the frequency band (F₀, F₁);two signals are formed, one of which is obtained by amplifying thesignal filtered in the frequency band (F₀, F₁) and limiting theamplitude thereof, and the second signal is the above filtered signal ora filtered signal linearly amplified without altering the shape thereof;the two signals thus obtained are multiplied; the resultant signal isfiltered in the frequency band [ΔF_(nar), (F₁−F₀)]; and the envelope ofthe signal obtained is separated and demodulated to obtain aninformation signal, wherein ΔF_(nar) is the frequency band of thesquared amplitude variation spectrum of the interference voltage.